Three-phase separation downhole

ABSTRACT

Three-phase separation is achieved downhole in a production well by forming an oil/liquid separation zone in the well between two packers which are positioned between producing and disposal zones. Gas is permitted to separate above the top packer. A pump is used to pump an oil/water mixture from above the top packer into the separation zone. Adequate residence time is permitted in the separation zone to achieve the separation. A check valve in the bottom packer is used to provide pressure to raise the oil to the wellhead. A control valve at the wellhead is used to control pressure in the separation zone and the relative sizes of the streams issuing therefrom.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/449,441,filed Aug. 7, 2009, now ______, which was a section 371 of PCTapplication number PCT/US2008/001698, filed Feb. 8, 2008, which claimedthe benefit of U.S. provisional application No. 60/900,468, filed Feb.9, 2007.

FIELD OF THE INVENTION

This invention relates to separating three phases downhole, for example,oil, gas and water, and producing separate streams of each.

BACKGROUND OF THE INVENTION

The disclosures of Michael et al, U.S. Pat. No. 7,150,315, issued Dec.19, 2006, and Cognata, US 2006/0027362, published Feb. 9, 2006, now U.S.Pat. No. 7,255,167 issued Aug. 14, 2007 are incorporated by referenceherein.

Hydrocarbon producing wells often produce mixtures of oil, gas andwater. The water must be safely disposed of. Often, it is stored insurface tankage on the wellsite and trucked away to a disposal well. Theprocedure causes great expense, and can cause the well to be shut in ifthe amount of hydrocarbons being produced does not generate sufficientprofits to cover the expense of water disposal. The heavy trucksinvolved also damage the secondary roads often found in oil and gasproducing areas.

There is a need for a system to reduce the amount of water which isproduced at the wellhead.

OBJECTS OF THE INVENTION

It is an object of this invention to provide a well production system inwhich three-phase well fluids which enter the well downhole areseparated downhole into three separate phases.

It is another object of this invention to provide a well productionsystem in which a separated water phase is injected into a disposal zonedownhole without ever being produced on the surface of the wellsite.

It is another object of this invention to provide a well productioncontrol system to insure that the produced oil stream at the wellheadcontains only minor amounts of water and that the water disposal streamdownhole contains only minor amounts of oil.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a method for controlling thedegree of oil/water separation achieved in a process for separatingoil/water/gas phases downhole. A mixture of oil, gas and water isproduced in a first section of a cased well. The gas is separated fromthe mixture and withdrawn from an upper end of the first section. Amixture of oil and water is collected at a lower end of the firstsection. The collected mixture of oil and water is pumped from the lowerend of the first section into a second section positioned beneath thefirst section. In the second section, the oil phase is separated fromthe water phase. The second section is maintained at adequate pressureto flow the oil phase to the wellhead and the oil phase flows up atubing to the wellhead to produce the oil. The produced oil is analyzedfor entrained water droplets. An electronic signal representative of theamount of water in the produced oil is produced. A valve operablyassociated with the tubing is then manipulated responsively to theelectronic signal to maintain the amount of water in the produced oilwithin predetermined lower and upper limits.

Another embodiment of the invention provides a technique for controllingthe pressure available downhole to produce the oil phase from anoil/gas/water separation conducted downhole. A mixture of oil, gas andwater is produced in a first section of a cased well. The gas isseparated from the mixture, the separated gas is withdrawn from an upperend of the first section, and a mixture of oil and water is collected ata lower end of the first section. The collected mixture of oil and wateris pumped from the lower end of the first section into a second sectionpositioned beneath the first section. A valve is positioned at the lowerend of the second section. The valve is set to provide adequate pressurein the second section to flow the oil phase to the wellhead. The oilphase is separated from the water phase in the second section and flowedup the tubing to the wellhead where it is produced.

Another embodiment of the invention provides a method for facilitatingoil/water separation in an oil/gas/water separation conducted downhole.A mixture of oil, gas and water is produced in a first section of acased well. The gas is separated from the mixture and the separated gasis withdrawn from an upper end of the first section. A mixture of oiland water is collected at a lower end of the first section. Thecollected mixture of oil and water is pumped from the lower end of thefirst section into a second section positioned beneath the firstsection. In the second section the oil phase is separated from the waterphase. The improvement is to establish an environment in the secondsection to facilitate phase separating the oil phase from the waterphase.

In a further embodiment of the invention, there is provided an apparatusfor the production of hydrocarbons. A wellbore is provided extendinginto the earth from a wellhead positioned at the surface of the earth. Acasing defined by a tubular casing sidewall lines the wellbore. A set ofproduction perforations extend though the casing sidewall at a firstlongitudinal position and establish communication between a hydrocarbonproduction zone and an inside of the casing. A set of disposalperforations extend through the casing sidewall at a second longitudinalposition establishing communication between an inside of the casing anda waste water disposal zone. The set of production perforations isspaced apart from the set of disposal perforations and is positionedbetween the set of disposal perforations and the wellhead. A pumpassembly is positioned inside of the casing and sealed against thetubular casing sidewall at a seal location between the first set ofperforations and the second set of perforations. The pump assembly hasan intake positioned to draw fluid from the inside of the casing abovethe seal location and a discharge to discharge fluid into the inside ofthe casing below the seal location. A packer is positioned inside of thecasing between the pump assembly and the disposal perforations. Thepacker has a passage extending through it. A separation chamber isdefined in the casing between the pump and the packer. A first valve isoperatively associated with the passage extending through the packer. Atubing extends from an upper end of the separation chamber to thewellhead. The tubing forms a flow path between the separation chamberand the wellhead. A second valve is operatively associated with thetubing. The apparatus can be used for carrying out the methods asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematically shows an overview of one system according to theinvention.

FIG. 1 b schematically shows an alternative portion to a FIG. 1 asystem.

FIG. 2 schematically shows details of a pump according to an embodimentof the invention.

FIG. 3 schematically shows further details of the pump in FIG. 2.

FIG. 4 a schematically shows additional details of a portion of a pumpas shown in FIGS. 2-3 and 4 b.

FIG. 4 b schematically shows additional details of a portion of a pumpshown in FIGS. 2 and 3.

FIG. 5 schematically shows additional details of the system shown inFIG. 1 a.

FIG. 6 schematically shows features of a control system according to anembodiment of the invention.

FIG. 7 schematically shows additional features of a control systemuseful in conjunction with the system of FIG. 5.

FIG. 8 schematically shows a control logic diagram useful in conjunctionwith the control systems of FIGS. 5 and 6.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the invention, there is provided an apparatus forthe production of hydrocarbons. A wellbore 11 is provided extending intothe earth from a wellhead positioned at the surface of the earth. Acasing 10 defined by a tubular casing sidewall lines the wellbore. A setof production perforations 31 extend though the casing sidewall at afirst longitudinal position and establish communication between ahydrocarbon production zone A and an inside of the casing. A set ofdisposal perforations 41 extend through the casing sidewall at a secondlongitudinal position establishing communication between an inside thecasing and a waste water disposal zone B. The set of productionperforations is spaced apart from the set of disposal perforations andis positioned between the set of disposal perforations and the wellhead.A pump assembly 25 is positioned inside of the casing and sealed againstthe tubular casing sidewall at a seal location 40 between the first setof perforations and the second set of perforations. The pump assemblyhas an intake 30 positioned to draw fluid from the inside of the casingabove the seal location and a discharge 50 to discharge fluid into theinside of the casing below the seal location. A packer 100 is positionedinside of the casing between the pump assembly and the disposalperforations. The packer has a passage extending through it. Aseparation chamber D is defined in the casing between the pump and thepacker. A first valve 90 is operatively associated with the passageextending through the packer. A tubing 120 extends from an upper end ofthe separation chamber to the wellhead. The tubing forms a flow pathbetween the separation chamber and the wellhead. A second valve 105 isoperatively associated with the tubing.

In a preferred embodiment of the invention, a control system 175 isoperatively associated with the second valve for opening the secondvalve in response to a decrease in water concentration in fluid carriedby the tubing and closing the second valve in response an increase inwater concentration in fluid carried by the tubing.

The first valve 90 preferably unseats in response to a pressuredifference across the first valve for flow from the separation chamber.If desired, a coagulation media can be positioned in the second chamber.

The pump assembly 25 preferably comprises an outer housing 20, a pumptraveling barrel 33, and a pump pull rod 102. The outer housing ispositioned in the well casing 10, an annulus 21 being defined betweenthe well casing and the outer housing. The pump traveling barrel ispositioned in the outer housing and has an upper end and a lower end.The pump traveling barrel is partly closed at its upper end by anannular wall 67. The pump pull rod has a portion positioned in the pumptraveling barrel and a portion extending from the upper end of the pumptraveling barrel. The pump pull rod has a lower end and a radiallyoutwardly extending wall 69 from its lower end which seals against theannular wall at the upper end of the pump traveling barrel when the pumppull rod is pulled upwardly. The pump pull rod further has at least oneradial outward protrusion spaced apart from its lower end to urge thetraveling barrel downwardly. A first pump chamber 76 is defined insideof the traveling barrel and a second pump chamber 79 is defined outsideof the pump traveling barrel between an upper end of the pump travelingbarrel and an upper end of the outer housing, an annulus for downflow offluid being defined between the outside of the traveling barrel and theinside of the outer housing. Upstroke on the pull rod expands the firstpump chamber and draws fluid in through the pump intake, whilesimultaneously compressing the second pump chamber and forcing fluiddown the annulus between the outside of the traveling barrel and thehousing and out the pump discharge E.

In the illustrated embodiment, the pump assembly comprises a secondpacker 40 sealed against the tubular casing sidewall and a bottom sealmanifold 30 carried by the second packer. The bottom seal manifolddefines both the pump intake and the pump discharge ports. An axial pulltube 11 extends upwardly from the bottom seal manifold. An inside of theaxial pull tube is in flow communication with the pump intake. The axialpull tube has an upper end forming a valve face and at least one radialport 66 spaced apart from the upper end forming a flow path between theinside of the axial pull tube and an annulus between the outside of theaxial pull tube and the inside of the pump traveling barrel. An annularseal 77 is mounted to an outside surface of the axial pull tube slidablysealingly engaged with an inner surface of the pump traveling barrel. Anannular standing valve 44 is mounted to an inside of the pump travelingbarrel for sealing against the valve face at the upper end of the axialpull tube when the pump traveling barrel is urged down.

Preferably, a drop tube 50 extends downwardly from the bottom sealmanifold in flow communication with the pump discharge. The drop tubeforms a discharge into the separation zone. Also, the tubing 120extending from an upper end of the separation chamber to the wellhead ispreferably located along an axis of the wellbore. FIG. 1 is an overviewof the invention. The invention is applicable to a production wellhaving a suitable producing zone separated a distance above a suitabledisposal zone. The well is cased in the usual way. Casing perforationsestablish flow communication with the producing zone. Casingperforations are provided to establish flow communication with thedisposal zone.

A packer 40 separates the producing zone from the disposal zone. Thecasing contains adequate volume between the producing zone and thepacker 40 to permit any gases introduced into the casing from theproducing zone to separate from any liquids introduced into the casingfrom the producing zone. If desired, emulsion breaking chemicals can beintroduced into the volume to facilitate oil/water phase separation inthe oil/water separation chamber situated beneath the packer 40.

A pump 20 is mechanically attached to the packer 40. Although electricpumps could be employed in the invention, a mechanical pump ispreferred, to reduce shear which contributes to emulsification. The pumpis preferably operated by a sucker rod string extending from a motor atthe wellhead. The power stroke is preferably the upstroke, to avoidexcessive wear or possible buckling of the sucker rod string.

As shown in FIG. 4, the oil/water mixture enters the pump through inletsin a bottom manifold 30 positioned adjacent the packer. An inner oiltube passes axially through the pump from the separation zone to thewellhead. A pull tube 11 is positioned radially outward from the inneroil tube and mechanically connected to the bottom manifold 30. As shownby FIG. 5, an outer barrel 20 is positioned radially outward from thepull tube and is mechanically attached to the sucker rod by means of thepull rod. A pump chamber is defined radially from the inner oil tube tothe outer barrel, and axially from the standing valve 44 to the upperlimit of the outer barrel where it seals against the pull rod. The pumpchamber expands and fills during the upstroke, simultaneously forcingfluid existing outside the pump chamber downward through the ports inthe bottom in the bottom manifold into the separation chamber. On thedown stroke, the seal between the traveling barrel and the pull rod isunseated, the standing valve closes, allowing the fluid within the pumpchamber to exhaust upward outside of the pump chamber, and preventingthe flow of fluid inside the pump chamber downward through the pulltube. The fluid exits the downflow annular chamber via passages throughthe bottom manifold 30 and flows into a pump discharge tube extendingdownwardly through the packer and extending into the separation chamber.The pump discharge tube preferably extends into the separation zoneseveral feet, and is provided with apertures through its sidewall and issealed at its bottom end to introduce fluids into the separation zonewithout introducing a lot of turbulence. If desired, the discharge tubeand/or annulus between the discharge tube and the casing can be packedwith an oil coagulation media, such as a high surface area materialformed from polyvinylchloride, polypropylene, or stainless steel.

In operation, an oil phase forms in an upper part of the separationchamber, and a water phase forms in a lower part of the chamber. The oilphase flows from the upper portion of the chamber by the inner tuberunning through the pump, via one or more inlets opening through thesidewall of the pump discharge tube, by virtue of back pressure causedby the pressure relief valve positioned in the packer at the bottom ofthe separation chamber. The inner tube generally has an inside diameterin the range of 0.5 inches to 1 inch, to provide adequate flow.

As shown in FIG. 6, a flow restrictor is associated with the inner tubeat its upper end. By restricting flow at this point, pressure can beraised in the separation zone which is adequate to unseat the checkvalve and permit water flow from the bottom of the separation chamber tothe disposal zone. Conversely, by opening the restriction, oil flow canbe increased, and water flow can be decreased.

To provide automated operation, the oil line at the wellhead is providedwith a device to measure the concentration of water contained in theoil, or vice versa. Suitable devices are known. For example, a Red-Eye(R) water cut meter with transmitter is suitable. This device measuresthe concentration of oil by employing infrared principles. A signalrepresentative of the amount of water in the oil is produced, comparedto a set point signal, and a manipulation signal produced in response tothe comparison to control. A motor valve is suitably employed as therestriction device for this purpose. By manipulating the valve so as tomaintain a small amount of water in the oil, excessive water productionat the wellhead, or loss of oil into the disposal zone, can be avoided.

FIG. 1 is a downhole system overview. Oil, gas and water are producedfrom the production zone A. All three phases enter into the annulusbetween the casing 10 and tubing 20. Sufficient time is provided betweenthe three phase mixture entering the casing-tubing annulus, and when themixture enters the barrel seal manifold 30 (BSM 30), to allow the gas tonaturally separate and flow to the surface. The two-phase oil and watermixture is pumped past the first packer 40 and exits below it at 50. Bymeans of natural (gravity) separation, the oil and water segregate into3 distinct sections, oil only 60, oil and water 70 and water only 80. Aback pressure valve 90 is utilized to ensure sufficient pressure betweenthe first packer 40 and the second packer 100 to drive the oil into thelower BSM 110, up the inner tube 120, and on to the surface. Afterpassing through the back-pressure valve 90, the water is driven into thedisposal zone B.

Alternatively, as shown in FIG. 1 a, the drop tube 50′ can be connecteddirectly to second packer 100′ which carries backpressure valve 90′.

FIG. 2 is a pump detail. FIG. 2 shows the oil and water mixturetraveling down the casing-tubing annulus 21. The oil and water enterinto the BSM 30 and are drawn into the pump 25.

FIG. 3 is a pump detail. As shown in FIG. 3, the fluid mixture thentravels up the pump pull tube 11 and into the volume formed inside thepump traveling barrel 33.

FIG. 4 is a pump detail of a pump fill cycle. When the fluid mixture hastraveled into the pull tube 11 from the BSM 30, it enters the pumpvolume. As the traveling barrel 33 moves upward, the standing valve 44opens. The upward movement of the standing valve is limited by thestanding valve stop 55. The fluid inside the pull tube travels past thestanding valve, via the pull tube ports 66, and enters the volumecreated inside the pump during the upstroke. The annular area betweenthe pull tube and the traveling barrel is sealed 77 below the pull tubeports. During the downstroke, the standing valve closes, seating thesealing against the pull tube. This seal prevents fluid within thetraveling barrel from traveling back into the BSM 30.

FIG. 5 is a pump detail of a discharge cycle. During the upstroke, thetraveling barrel 33 seats against the pull rod 102 and forms a seal,preventing flow from the outside of the traveling barrel to the insideof the traveling barrel. The pump forces the fluid in the volume betweenthe traveling barrel and the tubing 20 down through the bypass chambersin the BSM. The fluid is prevented from traveling up the tubing by thedownhole stuffing box (DSB) 54, which seals between the outer surface ofthe pull rod and the tubing. During the downstroke, the traveling barrelunseats from the pull rod, the standing valve closes (FIG. 4), and thefluid from inside the traveling barrel is displaced to the outside ofthe traveling barrel. The annular area between the inside of the pullrod and the inner tube is sealed 65.

FIG. 6 shows the relationship between production zone A, disposal zoneB, oil intake C, separation chamber D, and pump discharge E.

FIG. 7 illustrates a control system. The pump discharge E enters intothe separation chamber D. As the oil and water separate, three distinctzones form in the separation chamber. Oil on top, water on bottom, and acenter zone where it is mixed. The pressure at the pump discharge E isequal to the disposal zone B pressure plus the pressure set on thebackpressure check valve 90. The pressure differential between the pumpdischarge pressure and the variable restrictor 105 provides the drivingforce to flow oil to the surface. The flow rate of the oil to thesurface is changed by varying the surface restrictor.

In FIG. 8, pump discharge E is provided at flow rate Q_(P) and pressureP_(P). Separation zone D contains oil phase 60, mixture layer 70, andwater phase 80. Check valve 90 controls water discharge to disposal zoneB at flow rate Q_(W) and pressure P_(W). Oil production is regulated byrestrictor 105, which is controlled by motor valve M. Motor valve M iscontrolled by device 155 to measure oil in water and produce arepresentative signal, with controller 165 to receive the representativesignal and vary the signal to motor valve M.

Given sufficient separation chamber volume, the presence of a percentageof water in the oil produced to the surface indicates the relativeposition of the oil intake to the oil/water interface (mix zone). As theoil/water interface moves upward in the chamber, the amount of waterpresent in the oil will increase. Given this relationship, a controlscheme as shown in FIG. 9 will ensure that sufficient oil is produced tothe surface. In FIG. 9, signal 200 designates “r=motor signal”, box 210designates “motor manipulates restrictor”, box 220 designates “pressuredeveloped at restrictor”, box 230 designates “Q_(O) (oil flow)”, line240 is “oil produced”, and box 250 is “presence of water in oilproduced”. The percentage of water present in the oil produced to thesurface provides the feedback for the system. The restrictor ismanipulated, via a motor, which changes the flowrate of oil to thesurface, thereby influencing the level of the oil/water interface, andchanging the percentage of water present in the oil produced to thesurface. The process is repeated until the target percentage of water isreached.

The system operates such that only water is pumped into the disposalzone. Two limiting conditions must be satisfied for this system tooperate correctly. First, the separation chamber must be large enough toallow sufficient time for the fluid being pumped into it to separateinto oil and water. This requirement can be expressed as:

V _(CHAMBER) =S.F.×Q _(PUMP) ×T _(RETENTION)

Where

-   -   V_(CHAMBER)=Volume of Separation Chamber    -   S.F.=Safety Factor    -   Q_(PUMP)=Flowrate of fluid discharged from the pump    -   T_(RETENTION)=Retention Time (time required for oil to separate        from the water)

Secondly, the flowrate of the oil to the surface must be sufficient sothat the oil does not build up in the chamber, causing the oil/waterinterface (mix zone) to migrate low enough so that oil is pumped to thedisposal zone. The ratio f oil produced to the surface, to waterdisposed, must equal the ratio of oil to water entering the separationchamber. For a given pumping rate, the flow rate of the oil must becontrolled via a manipulation of the variable restrictor at the surface.This can be expressed as:

Q _(PUMP) =Q _(OIL) +Q _(WATER)

Q _(OIL) =C×Q _(PUMP)

Where

-   -   Q_(WATER)=Flowrate of water to disposal zone    -   Q_(OIL)=Flowrate of oil produced at surface    -   C=(volume of oil/volume of total fluid) as discharged from the        pump    -   Q_(PUMP)=Flowrate of fluid discharged from the pump

Furthermore, “C” and Q_(PUMP) are known. “C”, also referred to as “cut”,is a function of the reservoir (production zone) and “Q_(PUMP)” isdetermined from the pumping parameters.

One embodiment of the invention provides a method for controlling thedegree of oil/water separation achieved in a process for separatingoil/water/gas phases downhole. A mixture of oil, gas and water isproduced in a first section of a cased well. The gas is separated fromthe mixture and withdrawn from an upper end of the first section. Amixture of oil and water is collected at a lower end of the firstsection. The collected mixture of oil and water is pumped from the lowerend of the first section into a second section positioned beneath thefirst section. In the second section, the oil phase is separated fromthe water phase. The second section is maintained at adequate pressureto flow the oil phase to the wellhead and the oil phase flows up atubing to the wellhead to produce the oil. The produced oil is analyzedfor entrained water droplets. An electronic signal representative of theamount of water in the produced oil is produced. A valve operablyassociated with the tubing is then manipulated responsively to saidelectronic signal to maintain the amount of water in the produced oilwithin predetermined lower and upper limits.

Another embodiment of the invention provides a method for controllingthe pressure available downhole to produce the oil phase from anoil/gas/water separation conducted downhole. A mixture of oil, gas andwater is produced in a first section of a cased well. The gas isseparated from the mixture, the separated gas is withdrawn from an upperend of the first section, and a mixture of oil and water is collected ata lower end of the first section. The collected mixture of oil and wateris pumped from the lower end of the first section into a second sectionpositioned beneath the first section. A valve is positioned at the lowerend of the second section. The valve is set to provide adequate pressurein the second section to flow the oil phase to the wellhead. The oilphase is separated from the water phase in the second section and flowedup the tubing to the wellhead where it is produced.

Another embodiment of the invention provides a method for facilitatingoil/water separation in an oil/gas/water separation conducted downhole.A mixture of oil, gas and water is produced in a first section of acased well. The gas is separated from the mixture and the separated gasis withdrawn from an upper end of the first section. A mixture of oiland water is collected at a lower end of the first section. Thecollected mixture of oil and water is pumped from the lower end of thefirst section into a second section positioned beneath the firstsection. In the second section the oil phase is separated from the waterphase. The improvement is to establish an environment in the secondsection to facilitate phase separating the oil phase from the waterphase, for example, by introducing phase separation chemicals into thesecond section, or by positioning a coagulation media in the secondsection to facilitate separating the oil phase from the water phase.

What is claimed is:
 1. A method comprising producing a mixture of oil,gas and water in a first section of a well defined by a casing,separating the gas from the mixture, withdrawing the separated gas froman upper end of the first section, and collecting a mixture of oil andwater at a lower end of the first section, pumping the collected mixtureof oil and water from the lower end of the first section into a secondsection positioned beneath the first section at a pressure sufficient toflow an oil phase separated from the mixture to the wellhead,positioning a settable pressure-relief check valve at the lower end ofthe second section, setting the settable pressure-relief check valve toprovide adequate pressure in the second section to flow the oil phase tothe wellhead, phase separating the oil phase from a water phase in thesecond section, flowing the oil phase up a tubing to the wellhead, andproducing the oil.
 2. A method as in claim 1 further comprisingintroducing phase separation chemicals into the second section.
 3. Amethod as in claim 1 further comprising positioning a coagulation mediain the second section to facilitate separating the oil phase from thewater phase.
 4. A method as in claim 1 further comprising providing aset of disposal perforations through the casing sidewall at a positionbeneath the settable pressure-relief check valve, said set of disposalperforations establishing communication between an inside of the casingand a waste water disposal zone, flowing the water phase past thesettable, pressure-relief valve, and disposing of the water through theset of disposal perforations.
 5. A method as in claim 4 furthercomprising positioning a variable flow restrictor in the tubing at alocation on the surface near the wellhead for restricting fluid flowthrough the tubing sufficiently to unseat the settable pressure-reliefcheck valve and provide for the disposal of waste water through the setof disposal perforations.
 6. A method as in claim 5 further comprisingopening the variable flow restrictor in response to a decrease in waterconcentration in fluid carried by the tubing and closing the variableflow restrictor in response an increase in water concentration in fluidcarried by the tubing to maintain the concentration of water in thefluid within predetermined lower and upper limits.